Commonly used RTL-SDR dongles do not have the Temperature Compensated Crystal Oscillators (TCXO) so in settings you need to specify a value frequency correction (frequency offset in PPM) which is unique to each dongle. The program has the special option: --freq-correction <value>, for example, --freq-correction 62.
Oscillators and other frequency control devices specify their frequency variation in units of parts per million (PPM). For example, 62 PPM of 131.725 MHz represents a variation in frequency of 8166.95 kHz.
Use general software like SDR# and tune to a known signal like ACARS, ATIS or a trunking channel that you know the frequency of. Then adjust the PPM offset in SDR# so that your signal lines up with the known frequency. Record this PPM value and use it in acarsdeco2. Before operating make sure you let the dongle warm up for a ~ twenty-thirty minutes so the PPM stabilizes.

When using an high performance antenna, setting maximum gain (49.6) may be excessive. As a result, there is existing a risk of overloading the RF amplifier's stage with a strong signal.
I am suggesting that it would be better to determine the optimal value of required gain in the specific conditions. Typically, this value is in the range 25.4 - 38.6 (for dongle with Rafael Micro R820T/T2 tuner). The program has the special option: --gain <value>, for example, --gain 38.6.

If you enable using an additional data from external files (basestation.sqb, flightroute.sqb and the like) the question marks in the table will be replaced by the corresponding data about the aircraft.